Camshaft phasers for varying the valve timing of internal combustion engines are well known. A phaser typically comprises a rotor element that is attached to the end of a camshaft and is variably displaceable rotationally within a stator element driven by the engine crankshaft. Prior art phasers typically are actuated by pressurized oil that is derived from the engine's main oil supply and is selectively directed by electrically-controlled valving to chambers within the phaser to alter the phase relationship between the rotor and stator, and, hence, between the camshaft and crankshaft. Providing oil from the engine can require extensive and undesirable modification of the engine block and/or the camshaft and/or the forward camshaft bearing. Such modifications can be expensive and difficult to implement and are known to be important considerations when adapting a camshaft phaser to an existing engine design. It is highly desirable, therefore, in the art to provide a camshaft phaser requiring little or no modification to the receiving engine.
Further, prior art phasers experience a delay in oil pressure and flow upon start-up of a phaser-equipped engine, which prevents immediate phaser operation. To prevent uncontrolled phaser motion during this delay, vane phasers typically are equipped with additional locking devices to prevent rotor movement. Further, engine oil pressure decreases significantly for hot idle conditions, limiting phaser response rate. Further, oil quality and air entrainment may vary with time and operating conditions, which can also affect phaser torsional stiffness, resulting in unwanted holding position variation. Further, oil viscosity may vary over several orders of magnitude within the normal range of engine operating temperatures, causing a temperature-dependent variation in phaser response rate. And finally, it is difficult to achieve phase change rates greater than about 100 crank degree/second with engine-oil motivated phasers; faster phaser rates require significantly larger engine oil pumps, larger oil flow passages, and higher system pressures. Such changes are expensive in manufacture and adversely affect fuel economy.
A typical cam phaser in good working order exhibits a characteristic level of torque-imposed instability as a result, in part, of the rotor of the cam phaser being mounted directly to the engine camshaft. In opening an engine valve, the valve follower leaves the base circle portion of the cam lobe and begins to climb the rising edge of the eccentric portion, imposing a resistive (negative) torque on the camshaft. At some further position of the cam rotation, the resistive torque reaches a maximum, then declines to zero, and then becomes an assistive (positive) torque in the opposite direction as the follower descends the falling edge of the eccentric portion as the valve closes. Thus, for each rotation of the camshaft lobe, an oscillatory torque is imposed on the camshaft. For a multiple cylinder engine, this cycle is repeated several times for each revolution of the camshaft.
U.S. Pat. No. 6,453,859 B1 discloses a multi-mode control system for variable camshaft devices wherein torque-induced phaser oscillatory motion is employed for providing pressurized engine oil and/or phaser chamber oil to the advance and retard chambers of the phaser via a spool valve. A first drawback of the invention is that an engine oil supply is still required for actuation of the phaser, requiring modification of the associated engine block or head. A second drawback is that an adequate spool valve is relatively cumbersome and therefore has a relatively low reaction rate. A third drawback is that the system requires both a spool valve and a plurality of check valves, as does the apparatus disclosed in U.S. Pat. No. 5,645,017.
What is needed is method and apparatus for controlling actuation of a camshaft phaser without reliance on engine oil supply or pressure.
What is further needed is such method and apparatus wherein flow-controlling check valves are directly actuated without resort to an additional spool valve.
It is a principal object of the present invention to provide actuation control of a camshaft phaser without reliance on engine oil supply or pressure.
It is a further object of the present invention to provide a camshaft phaser which may be installed into an existing engine without a requirement for a real-time supply of engine oil to actuate the phaser.
It is a still further object of the invention to provide a camshaft phaser wherein flow-controlling check valves are directly actuated by a solenoid without requiring an additional spool valve.
It is a still further object of the invention to provide an improved camshaft phaser wherein inherent oil pressure differences within a stator are harnessed to alter the rotational position of a rotor within the stator.